HD74HC4538
Power-Down Considerations
Large values of CX may cause problems when powering down the HC4538 because of the amount of energy stored in
the capacitor. When a system containing this device is powered down, the capacitor may discharge from VCC through
the input protection diodes at pin 2 or pin 14. Current through the protection diodes must be limited to 30 mA;
therefore, the turn-off time of the VCC power supply must not be faster than t = VCC•CX/(30 mA). For example, if VCC
=
5 V and CX = 15 µF, the VCC supply must turn off no faster than t = (5 V)•(15 µF)/30 mA = 2.5 ms. This is usually not
a problem because power supplies are heavily filtered and cannot discharge at this rate.
When a more rapid decrease of VCC to zero voltage occurs, the HC4538 may sustain damage. To avoid this possibility,
use an external clamping diode.
VCC
R
X
2, 14
T2
C
X
VCC
M
1
Upper Reference
Circuit
Output Latch
M2
2k
+
–
M
3
Vref Upper
6, 10
7, 9
S
1
Q
Lower Reference
Circuit
VCC
M
4
Q
+
–
Trigger-Control Circuit
4, 12
5, 11
Vref Lower
A
B
C
Q
C
R
Trigger-Control
Reset Circuit
3, 13
C
D
Reset Latch
Figure 1. Function Diagram
Rev.2.00 Mar 30, 2006 page 6 of 13
RENESAS [ RENESAS TECHNOLOGY CORP ]
